The supernumerary cat eye syndrome (CES) chromosome is dicentric, containing two copies of 22pter→q11.2. We have found that the duplication breakpoints are clustered in two intervals. The more proximal, most common interval is the 450–650 kb region between D22S427 and D22S36, which corresponds to the proximal deletion breakpoint interval found in the 22q11 deletion syndrome (DiGeorge/velocardiofacial syndrome). The more distal duplication breakpoint interval falls between CRKL and D22S112, which overlaps with the common distal deletion interval of the 22q11 deletion syndrome. We have therefore classified CES chromosomes into two types based on the location of the two breakpoints required to generate them. The smaller type I CES chromosomes are symmetrical, with both breakpoints located within the proximal interval. The larger type II CES chromosomes are either asymmetrical, with one breakpoint located in each of the two intervals, or symmetrical, with both breakpoints located in the distal interval. The co-localization of the breakpoints of these different syndromes, plus the presence of low-copy repeats adjacent to each interval, suggests the existence of several specific regions of chromosomal instability in 22q11.2 which are involved in the production of both deletions and duplications. Since the phenotype associated with the larger duplication does not appear to be more severe than that of the smaller duplication, determination of the type of CES chromosome does not currently have prognostic value.

About 1% of individuals with autism or types of pervasive developmental disorder have a duplication of the 15q11-q13 region. These abnormalities can be detected by routine G-banded chromosome study, showing an extra marker chromosome, or demonstrated by fluorescence in situ hybridization (FISH) analysis, revealing an interstitial duplication. We report here the molecular, cytogenetic, clinical and neuropsychiatric evaluations of a family in whom 3 of 4 siblings inherited an interstitial duplication of 15q11-q13. This duplication was inherited from their mother who also had a maternally derived duplication. Affected family members had apraxia of speech, phonological awareness deficits, developmental language disorder, dyslexia, as well as limb apraxia but did not have any dysmorphic clinical features. The observations in this family suggest that the phenotypic manifestations of proximal 15q duplications may also involve language-based learning disabilities.